1. Field of the Invention
This invention relates to a magnetic recording medium of the coating type.
2. Background Art
Magnetic recording media of the coating type have a magnetic layer of ferromagnetic powder dispersed in a binder on a non-magnetic substrate. The binders used are generally classified into a thermosetting type and an electron beam curing type. Binders of the electron beam curing type are advantageous for large scale production because of possible on-line curing after coating and orientation, but are low in dispersion of ferromagnetic powder, less liable to calendering and other processing after coating, and not necessarily satisfactory in electromagnetic characteristics as compared with binders of the thermosetting type. They have the disadvantages of poor electromagnetic characteristics and high error rates especially in the case of high sensitivity digital recording as employed in digital audio tape recorders (DAT).
For increasing the dispersibility in binders of the electron beam curing type, it was proposed in the art to use electron beam-curable resins having various polar groups (Japanese Patent Application Kokai (JP-A) Nos. 40744/1982, 0745/1982, 29145/1983, 29146/1983, 100222/1983, 92421/1982, 05927/1983, 8126/1983, 79427/1983, 146440/1983, 63221/1985, 120765/1985, 922/1986, 77134/1986, 89207/1986, 6430/1987, 28925/1987, 42322/1987, 43830/1987, 84422/1987, 88135/1987, 95724/1987, 267827/1989, 40615/1987, 195720/1987, 67314/1992, etc.). These binders, however, did not achieve satisfactory improvements and attempts were made to use two or more electron beam-curable resins having a polar group in combination (JP-A 127225/1984 246917/1986, 75071/1988, 279420/1988, 49122/1989, 92926/1989, 144210/1989, 68713/1990, etc.). All these prior art combinations were unsuccessful in achieving a satisfactory improvement in dispersion.
On the other hand, various magnetic tapes include a backcoat layer on the rear surface thereof remote from the magnetic layer for improving run, durability, and storage property. One typical backcoat-forming resin used in the prior art is a thermosetting combination of nitrocellulose and urethane resin to be crosslinked with polyisocyanate. This is because the nitrocellulose/urethane system is fully rigid and highly effective for preventing blocking between the magnetic layer and the backcoat layer in a hot humid environment.
However, the thermosetting type backcoat layer of this nitrocellulose/urethane system is low in dispersion of carbon black which is added for antistatic purposes, resulting in a rough surface, which causes toughening of the magnetic layer surface due to topographic transfer to the magnetic layer side and roll tightening when the backcoat layer is thermoset in a roll form, which in turn, results in increased error rates especially in the case of high density magnetic recording as employed in digital audio tape recorders (DAT). Such inconvenience might be eliminated by using a backcoat layer of the electron beam curing type. The backcoat layer of the electron beam curing type is also advantageous for large scale production because of possible on-line curing after coating.
However, nitrocellulose which is modified to be sensitive to electron beams is very difficult to handle because it can be decomposed upon exposure to electron beams. Then one would attempt to use electron beam-curable vinyl chloride resins instead of the nitrocellulose, but these resins are less rigid. Rigidity may be increased by increasing the content of vinyl chloride resin. The increased content of vinyl chloride resin, in turn, leads to brittle coatings which are liable to abrasion during tape run, resulting in increased error rates due to dusting.
Furthermore, prior art magnetic recording media using an electron beam-curable resin as a binder in their magnetic layer and backcoat layer are generally manufactured by the following two methods. That is, the first method is by coating one surface of a non-magnetic substrate, typically tape with one of the magnetic layer and backcoat layer, irradiating electron beams thereto for curing, winding the tape as a roll, thereafter coating the opposed surface of the tape with the other one of the magnetic layer and backcoat layer, and irradiating electron beams thereto for curing. The second method is by coating and curing the backcoat layer to one surface of tape and simultaneously therewith, coating and curing the magnetic layer to the other surface of tape.
Where magnetic recording media are manufactured by these methods, however, especially the backcoat layer becomes less flexible and more brittle so that it experiences greater abrasion at the contact with guide pins during operation in the deck, leaving abraded debris which can adhere to the magnetic layer surface with increased errors.
Making investigations on the preparation method of magnetic recording media, especially the method of irradiating electron beams to various layers for curing in order to solve the above-mentioned problems, we have found that since all the prior art methods are designed such that the backcoat layer is coated and then cured by irradiating electron beams directly to the backcoat layer, curing reaction can overrun so that the coating becomes brittle or weak.
Also, when the magnetic layer, backcoat layer and undercoat layer are formed in magnetic recording media by coating, coating compositions for forming the respective layers are used. These coating compositions often contain pigments such as carbon black. One of the binders used for dispersing pigments such as carbon black in the coating compositions is an electron beam-curable resin. As previously mentioned, the electron beam-curable resin is advantageous for large scale production because of possible on-line curing after coating. In preparing such coating compositions, resins of the solvent type are generally used by taking into account the stability of electron beam-functional groups.
However, if one attempts to disperse carbon black or the like in such a resin of the solvent type, it is often difficult to achieve desired dispersity. For example, a dispersing technique using a conventional kneader is difficult to disperse carbon black or the like because of a viscosity drop during kneading. Dispersing techniques using a mill with glass beads, metal media or ceramic media are also proposed while these techniques are difficult to set proper conditions including a choice of solvent and tend to invite an increase of viscosity with the progress of dispersion eventually to disable dispersion.